In the liquid penetrant non-destructive inspection of metal surfaces for cracks and flaws, liquid penetrant compositions are used to aid in visual examination to find the location of discontinuities in the surfaces being examined. One of the types of liquid penetrant materials used is the type containing fluorescent dyes, viz., dye substances which produce a readily visually observable color indication upon exposure of the fluorescent dye material to what is commonly called "black light" (ultraviolet light). Such materials greatly enhance the observability to the unaided eye when the surface is properly prepared and care is taken to insure that the liquid penetrants are applied, developed and/or selectively removed from the smooth continuous surfaces to insure contrast between the dye which penetrates into the surface discontinuities or flaws as compared with the remaining smooth or unflawed surfaces being examined.
It has been discovered that the dye substances which are the subject of this invention possess a unique combination of selective fluorescent properties in non-polar and highly polar liquid media, that renders them very useful in the field of liquid penetrant inspection. Typical dyestuffs of this invention when irradiated with ultraviolet light exhibit an intense luminescence when dissolved in a substantially non-polar liquid medium and exhibit a weak or no luminescence when dissolved in a substantially highly polar liquid medium. It is apparent that the substantially non-polar medium may be a mixture of miscible substances, and that the highly polar medium may also be a mixture of miscible substances. Further, the substantially highly polar medium containing one or more alcohols and glycols may include water.
Thus, for example, typical dyestuffs of this invention when irradiated with ultraviolet light will exhibit an intense luminescence when dissolved in a substantially non-polar oily medium: for example, refined kerosene or other aliphatic and/or alicyclic hydrocarbons; whereas said dyestuff will exhibit a weak or no luminescence when dissolved in a substantially highly polar liquid medium: for example, lower molecular weight alcohols such as methanol, ethanol, and isopropanol; glycols such as ethylene glycol, propylene glycol, and glycerol; and liquid organic acids such as acetic or propionic acids.
I have further found that when an intensely luminescing solution in a low surface tension non-polar solvent has added to it a high surface tension highly polar solvent the intensity of the luminescence decreases as the polar solvent is added until the intensity becomes very weak or negligible. That is, the luminescence of the original solution can be selectively "quenched." The term "quenchable" as used herein in reference to fluorescent dye penetrants refers to the ability of the subject dyestuffs to selectively alter their fluorescence intensity depending upon the polarity of the liquid medium in which they are dissolved, thus obtaining the disappearance of fluorescence in the background (smooth or non-flawed) areas to enhance the visual contrast between the dye which has penetrated the surface flaws or discontinuities and the dye which is contained on the non-flawed or smooth areas. The different surface tension characteristics of the non-polar and highly polar liquid media enable selective differentiation between the flawed and unflawed areas. The use of the dye substances of this invention enables the liquid penetrant inspection to be conducted with enhanced visual contrast, yet does not require removal of the quenching solution or liquid media, nor does it complicate the procedural aspects of the liquid penetrant inspection.
One novel use of such a selectively quenchable luminescent composition containing the dyestuffs of the present invention is in an improved process for the detection of flaws, wherein the luminescent composition is quenched selectively in the non-defective background areas of the object under inspection.
The dye substances of this invention are trisubstituted 4-aminonaphthalimides having one substituent (R.sub.1) on the imide nitrogen, being a member selected from the group consisting of:
(a) alkyl groups having from two to twelve carbon atoms, PA1 (b) substituted alkyl groups having at least two carbon atoms, PA1 (c) phenylalkyl groups whose alkyl moiety has from one to four carbon atoms and PA1 (d) alicyclic rings having from five to six carbon atoms in the alicyclic ring; PA1 (a) alkyl groups having from one to twenty-two carbon atoms, PA1 (b) phenylalkyl groups whose alkyl moiety has from one to four carbon atoms, PA1 (c) alicyclic groups having up to twelve carbon atoms, PA1 (d) tetrahydrofuranylmethyl and PA1 (e) a single alkylene chain forming a single heterocyclic ring with the amino nitrogen;
The remaining two substituents (R.sub.2 and R.sub.3) are on the nitrogen atom of the 4-amino group, and are a member(s) selected from the group consisting of:
with the foregoing two provisos.
It has been discovered that if R.sub.1 has less than two carbon atoms, viz., R.sub.1 is methyl, that such substances do not display suitable solubility properties to enable their use in liquid penetrant inspection procedures.
It has also been discovered that if R.sub.2 and R.sub.3 together have less than four carbon atoms, that such substances do not display suitable solubility properties to enable their use in liquid penetrant inspection procedures.
Suitable exemplary substituents for the R.sub.1 position include, but are not necessarily limited to, the following: ethyl, n-propyl, n-butyl, 2-methoxyethyl, benzyl, 2-phenylethyl, cyclohexyl, and n-dodecyl. Suitable exemplary substituents at the R.sub.2 and R.sub.3 positions include, but are not necessarily limited to, the following: methyl, ethyl, n-propyl, n-butyl, benzyl, tetrahydrofurfuryl, isopropyl, cyclopentyl, cyclohexyl, cyclododecyl, and C.sub.20-22 H.sub.42-46, the corresponding secondary amine of which is "Kemamine S-190," a commercially available mixture of C.sub.20 to C.sub.22 secondary alkyl amines. In the case where R.sub.2 and R.sub.3 together with the nitrogen atom comprise a single heterocyclic ring, suitable exemplary ring substituents include, but are not necessarily limited to, piperidine and pyrrolidine.
Specific dye substances, within the purview of the aforementioned formula and having the desired combination of quenchable fluorescence and solubility are as follows: 4-methylbenzylamino-1,8-naphthalic acid-N-n-butylimide; 4-di-(n-propyl)amino-1,8-naphthalic acid-N-n-butylimide; 4-(1-piperidino)-1,8-naphthalic acid-N-n-butylimide; 4-(1-piperidino)-1,8-naphthalic acid-N-benzylimide; 4-methyltetrahydrofurfurylamino-1,8-naphthalic acid-N-n-butylimide; 4-di(n-butyl)amino-1,8-naphthalic acid-N-n-butylimide; 4-di(n-butyl)amino-1,8-naphthalic acid-N-cyclohexylimide; 4-di(n-propyl)amino-1,8-naphthalic acid-N-cyclohexylimide; 4-di(n-butyl)amino-1,8-naphthalic acid-N-benzylimide; and 4-di(n-propyl)amino-1,8-naphthalic acid-N-benzylimide. Other exemplary dyestuffs will be apparent from the TABLE following Example 5.
The liquid penetrant solutions, per se (absent quenching liquid media), can be prepared in accordance with known preparation procedures such as those indicated in the article entitled "Liquid Penetrant Inspection," pp. 20-44 of Vol. 11 of the American Society for Metals Metals Handbook, 8th Edition, published in 1976 and prepared by the American Society for Metals Handbook Committee.
The dyestuffs of this invention offer improved visual contrast between the discontinuous and continuous (flawed and unflawed) areas of the part being inspected when compared with known penetrant solutions.